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Mill and shim AR15 barrel for accuracy?


redwoods
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A so called best gunsmith wants to mill and shim a barrel for my brother for accuracy. Faxon 18" fluted barrel.  He also wants to install a piston system for him.

Anyway, has anyone heard of milling and shimming their AR15 barrel for accuracy?   I was thinking maybe heavy barrels for 1000 yard shooting.

This is not for competitions, but just for a first time AR15 my brother is building for a shooting for fun gun.

 

Thanks,

Randy

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He shouldn't have to mill the barrel. Shims are normally used to line up the barrel nut, so that the gas tube is aligned with the receiver. I have squared the face of my AR uppers and then used loctite to bed the barrel in the receiver. Here's some info on squaring the receiver face.

 

https://www.bexararms.net/blog/receiver-face-squaring/

 

Here's the tool (video)

 

 

 

 

 

Edited by 157Beck
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He's probably talking about milling the receiver face square and wrapping the barrel extension in shim stock for the tightest fit possible. It's not a bad idea and won't hurt anything but 99 in 100 shooters won't be able to tell the difference unless his particular barrel or upper is horribly out of spec.

The weird thing is that the gunsmith wants to fit everything for accuracy but recommends a piston system. Pistons are not synonymous with accuracy.

Edited by TonytheTiger
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On 3/18/2020 at 3:32 PM, TonytheTiger said:

He's probably talking about milling the receiver face square and wrapping the barrel extension in shim stock for the tightest fit possible. It's not a bad idea and won't hurt anything but 99 in 100 shooters won't be able to tell the difference unless his particular barrel or upper is horribly out of spec.

The weird thing is that the gunsmith wants to fit everything for accuracy but recommends a piston system. Pistons are not synonymous with accuracy.

Lap and shim good for accuracy, piston system opposite effect. I would look elsewhere like quality factory built rifle.

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You want the face of the receiver to be flat and mate up well with the barrel extension ring. A quality upper receiver should already be 99% there but five minutes with the Brownells or similar lapping tool gets it 100% so why not? Then you want the barrel and receiver to fit together tightly and you can use a variety of methods individually or in combination. I can't imagine needed to wrap a shim around the barrel unless something is out of spec but I have noticed some barrels slide right in easily and others need persuasion with a wooden block. The sexiest way is to freeze the steel barrel and heat up the aluminum receiver and put them together quickly before they normalize (freeze barrel for a couple hours, bring it out, heat up receiver with a Berzomatic torch for 30 seconds all around the mounting area, slide together ASAP as the heat cools faster than the barrel warms). If the barrel is still a bit loose then add some green (expanding!) Loctite or if really bad a piece of thin metal shim stock (never seen this in my experience, makes me think something's wrong). 

 

The shims most people refer to are like very thin washers that go between the receiver's threads and the barrel nut so that the notches in the barrel nut can be timed to line up with where the gas tube runs. You want to avoid having the gas tube touch the barrel nut lest it impart a (very) slight influence on the free floating barrel. Lots of handguards come with these shims or you can buy an assorted package for $5. Somebody will mention they could affect accuracy but now you'd really be splitting hairs. 

 

Not many people here are going to support piston systems but the sales pitch convinces a lot of people too. Also consider if you'll eventually want to use a suppressor, etc. 

 

I don't know who does superior factory built rifles outside of JP Rifles but it should be within the skill set of a careful amateur assembler. Knowing what parts work well together and make economic sense is harder than actually putting together the rifle. I think the usual progression is someone buys a moderately priced rifle, reads enough gunporn and then wants to modify or build a nicer gun as their "good gun". Or if you have the money buy one good to go from JP or another established brand name. 

Edited by Frankly
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There are many surfaces that need to be square with each other when fitting the barrel. The receiver face, the threads, the 2 barrel flange surfaces that are clamped and the threads and inner surface of the retaining nut. If you cannot measure all of these to see if there is a problem in how they relate to each other then you should probably leave them alone. Way too much to go wrong. Get quality components and assemble them. 

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